Recent geological work within the Contract Area of P.T. Riotinto Bethlehem Indonesia in West Sumatra, including aerial photographic and aeromagnetic surveys by Lockwood Survey Corporation, have yielded much new data on the Sumatran Fault Zone and have revealed its intimate connection with volcanism. Three major fault zones each characterized by complex wrench, thrust and subordinate normal faulting can be recognized from aerial photographic interpretation. During the Quaternary significant di splacements along the Sumatran Fault Zone occurred and are still continuing, and in consequence morphological features and especially drainage patterns are in the closes t correspondence to the fracture systems. Strong E-W magnetic lineaments thought to represent major intermediate intrusives at depth are found associated with volcanic centres. Where these lineaments are cut by strands of the fault zones volcanoes are located. Igneous activity along original E-W a lignments and dextral tranSCUlTent faulting along NNW trending zones interact to produce a barrier range of volcanic rocks along the active fault zone with evenly spaced active volcanic centres at 75-100 km. distances along it. The total horizontal shift along the fault is approximately 130 km. since the inception of the present volcanic cycle. The fault system is thought to be a fundamental fracture whose full explanation is to be sought within the concept of a plate tectonic interpretation of the geology of Southeast Asia. 12 0 Volcano wilh hiltorlcal .rocord 01 activity. } From Mop by B. G. Esdl!r (111511) \'flth odditions from Von B.mm.Hn (1949) Alter. J. Kat~r. 1969. Figur •. I .